1. Field
The present invention relates generally to a method and apparatus for synchronizing a multiple access system. More specifically, the present invention relates to a novel and improved method and apparatus to synchronizing a Code Division Multiple Access (CDMA) system, an Orthogonal Frequency Division Multiplexing (OFDM) system or a hybrid of two such systems.
2. Background
In the prior art, either CDMA searcher information or Orthogonal Frequency Division Multiplexing (OFDM) channel estimation information is used for synchronization. A CDMA searcher looks for sectors and paths. By identifying the arrival time of various paths from various sectors, the CDMA searcher correlates the received signal with the pilot signal. Paths can be differentiated based on the differences in chip delay. i.e., different paths have different delays.
With ODFM channel estimation, all of the paths are represented by an impulse response, ^h. Channel estimates may be used to combine the paths. The channel delay spread is the span or width of the impulse response which equals the difference between the maximum and minimum path delays.
OFDM is a multicarrier transmission technique, which divides the available spectrum into many carriers. High-speed data signals are divided into tens or hundreds of lower speed signals. These lower speed signals are transmitted in parallel over respective frequencies within a Radio Frequency (RF) signal that are known as sub-carrier frequencies (“sub-carriers”). Each subcarrier is modulated by one of the low rate data streams. Thus, the ODFM signal is a sum of many signals with different subcarrier frequencies.
In addition, all of the carriers are orthogonal to one another. Because the carriers are orthogonal, each carrier has an integer number of cycles over a symbol period. Due to this, the spectrum of each carrier has a null at the center frequency of each of the other carriers in the system. See FIG. 1. As a results, there is minimal interference between the carriers, allowing then to be spaced as close as theoretically possible.
A discrete Fourier transform is used for baseband processing of the OFDM signal. The discrete Fourier transform is implemented as an Inverse Fast Fourier Transform (IFFT) and a Fast Fourier Transform (FFT) that modulates and demodulates parallel data blocks, respectively. The set of subcarriers generated during one transform defines an OFDM symbol. The subcarriers are serially transmitted over the channel of the time samples generated by the FFT. The duration of the OFDM symbol, which is the same as that of the subcarrier symbol, is thus equal to the time window of the transform.
Generating an OFDM signal in this manner ensures that the OFDM signal will retain its subcarrier orthogonality property when transmitted through a nondispersive channel. However, most channels are dispersive. Thus, significant time and/or frequency dispersion are introduced into the transmitted signal. These impairments introduce Intercarrier Interference (ICI) and Intersymbol Interference (ISI) and which can destroy the orthogonality of the subcarriers.
To protect against time dispersions including multipath, a guard interval equal to the length of the channel impulse response is introduced between successive OFDM symbols. The cyclic extended OFDM symbol thus consists of a guard interval and a useful part in which information is transmitted. The guard interval is commonly implemented by cyclic extension of the IFFT output (i.e., cyclic retransmission of part of the periodic transform). To maintain transmission efficiency, system designers typically endeavor to limit the guard interval to less than one quarter of the useful OFDM symbol duration.
FIG. 2 is a block diagram of a typical OFDM transmitter according to the prior art. An OFDM transmitter 10 receives a stream of baseband data bits 12 as its input. These input data bits 12 are immediately fed into an encoder 14. Encoder 14 typically uses a block and/or convolutional coding scheme to introduce error-correcting and/or error-detecting redundancy into the input bits 12.
In an OFDM communication system, data is communicated in a series of time domain bursts. To form each time domain burst, an IFFT is applied by a Digital Signal Processor (DSP) 16 to a group of frequency domain symbols producing digital time-domain OFDM symbols. DSP 16 also performs additional spectral shaping on the digital time-domain OFDM symbols. In addition, it adds a cyclic prefix or guard interval
Next, the digital time-domain OFDM symbols are converted to an analog signal in a digital-to-analog converter 20 and then converted to an Intermediate Frequency (IF) in IF transmitter 42. This composite IF OFDM signal is then passed into Radio Frequency (RF) transmitter 40 which then upconverts the IF OFDM signal to a desired selectable carrier frequency prior to final amplification and propagation across a transmission medium. Upconversion is typically achieved by mixing the IF signal with a variable frequency oscillator signal. The carrier frequency is varied by varying the oscillator frequency.
FIG. 3 illustrates that an OFDM signal is transmitted as blocks of user data separated by guard intervals known as cyclic prefixes. A cyclic prefix is a copy of a portion of an adjacent block of user data and is used to reduce Inter-Symbol Interference (ISI) caused by multipath fading. More particularly, only cyclic prefixes, as opposed to user data, are affected by ISI, as is known by those skilled in the art. Thus the removal of cyclic prefixes by an OFDM receiver removes the effects of ISI from the received OFDM signal.
In the prior art, CDMA systems have synchronized using CDMA searcher information, while ODFM systems have achieved synchronization using discrete frequency-domain channel estimator information. A CDMA searcher uses Pseudo Noise (PN) correlation to identify multipaths and multiple sectors. An OFDM channel estimator uses the frequency response at the pilot tone locations to identify the channel impulse response. However, a CDMA searcher is less effective when detecting weak or low power signals. If a multipath component is not detected, it will not be included in the signal demodulation and interference suppression. As a result, the signal-to-noise ratio will be lower. On the other hand, OFDM channel estimation suffers from aliasing if the channel delay spread exceeds the number of pilot tones. This is the case even if the pilot tones of adjacent OFDM symbols are staggered, so as to admit channel estimation whose length is twice the number of pilot tones.